The present invention relates generally to optical networks, and more particularly, to a multi-direction variable optical transceiver.
The following prior documents referenced in the background discussion, not material to patentability of the claimed invention, nevertheless provide additional information.
Super-Channel Experiment:
                [Xia1] T. J. Xia, G. A. Wellbrock, et al., “Field Experiment with Mixed Line-Rate Transmission (112-Gb/s, 450-Gb/s, and 1.15-Tb/s) over 3,560 km of Installed Fiber Using Filterless Coherent Receiver and EDFAs Only”, Proc. OFC-NFOEC, paper PDPA3, 2011.        [Xia2] T. J. Xia, G. A. Wellbrock, et al., “10,000-km Enhanced Long-Haul Transmission of 1.15-Tb/s Superchannel using SSMF only”, Proc. OECC, paper PDP-4, 2011.        [Huang1] Y.-K. Huang, M.-F. Huang, et al., “High-Capacity Fiber Field Trial Using Terabit/s All-Optical OFDM Superchannels With DP-QPSK and DP-8QAM/DP-QPSK Modulation”, Journal of Lightwave Technology, Vol. 31, No. 4, 2013.Variable Transmitter:        [Huang2] Y.-K. Huang, E. Ip, et al., “Terabit/s Optical Superchannel with Flexible Modulation Format for Dynamic Distance/Route Transmission”, Proceedings of OFC-NFOEC 2012, paper OM3H4, 2012.        [Liu] L. Liu, H. Y. Choi, et al., “First Proof-of-Concept Demonstration of OpenFlow-Controlled Elastic Optical Networks Employing Flexible Transmitter/Receiver”, Proceedings of PS 2012, PDP, 2012.Multi-Tone Generation:        [Yu] J. Yu, D. Qian, P. N. Ji, and T. Wang, “Generating a 400-Gbit/s single-channel optical signal”, U.S. patent application Ser. No. 12/789,943, filed May 28, 2010. (NEC Labs IR #09017).        [Ji] P. N. Ji, Y.-K. Huang, T. Wang, “Variable rate optical transmitter based on all-optical OFDM super-channel technology”, NEC Labs IR 11033, patent application Ser. No. 13/588,425.        
To meet the capacity demand, per-channel data rate in the WDM (wavelength division multiplexing) optical system has increased from 10 Gb/s to 40 Gb/s to 100 Gb/s. For 40 Gb/s and 100 Gb/s system, the conventional OOK (on-off keying) intensity modulation used in 10 Gb/s or below systems is no longer effective. New technologies, such as advanced modulation formats (such as QPSK, 8QAM, OFDM), multiplexing scheme (such as polarization multiplexing), receiving scheme (such as differential detection and coherent detection), and digital signal processing (such as chromatic dispersion compensation, fiber nonlinearity compensation, advanced error correction coding), are employed. For example, the most common technology for 100 Gb/s per channel DWDM long haul transmission system is polarization multiplexed (DP) QPSK modulation with digital coherent receiver.
As the capacity demand continues, beyond 100 Gb/s per-channel systems have also been researched and demonstrated. Those advanced technologies listed above might no longer sufficient in these systems, due to factors such as the bandwidth limitation of electronic components (such as DSP, ADC and DAC) and opto-electronic components (such as modulator, photo-detector). Optical super-channel scheme has been shown to be a good solution to solve these limitations. In an optical super-channel, instead of having a single optical carrier (such as in the 10 Gb/s, 40 Gb/s and most 100 Gb/s per-channel systems), multiple optical carriers (called subcarriers) are used. These subcarriers (also called optical tones) are usually generated from a single light source through a multi-tone generator (for example, using phase modulator or slow light laser), therefore maintain phase locked relationship. However in some broader definitions, different light sources can be used for different subcarriers.
A super-channel transceiver is used to transmit and receive optical superchannels. Similar to other optical transceivers, it consists of (1) the transmitter part, which converts multiple parallel data streams into an optical super-channel, and (2) the receiver part, which converts the received optical signal to multiple parallel data streams (FIG. 1(a)). In some implementations, the transmitter input is a single data input with higher data rate, which is then split into multiple parallel lower rate ones, and similarly the multiple data streams received by the receiver are merged into a larger rate data stream (FIG. 1(b)).
In a superchannel transmitter (such as in Ref. [Xia1], [Xia2], [Huang1]), these unmodulated optical subcarriers from multi-tone generator are modulated individually in parallel with different data, and the modulated subcarriers are then combined to form one superchannel. Additional optical or electrical signal processing, such as digital Nyquist filtering, optical shaping, etc., can be implemented on each subcarrier to obtain an almost rectangular frequency spectrum with a very small bandwidth close or equal to the Nyquist limit for inter-symbol interference-free transmission for each subcarrier and thus improve the transmission performance. FIG. 2 shows the schematic of a super-channel transmitter with a single light source and with optional DSP at each subcarrier. At the receiver, one or multiple or all subcarriers are received together, depending on the bandwidth of the photo-detector and related electronic hardware (such as DAC and DSP). If not all subcarriers are received together, the input optical signal can be split into different paths and each received by a separate receiver unit. These receiver units operate in parallel. At the input of these receiver units, there can be an optical filter to select the spectral band that need to be received, but if coherent receiver is used, the filter is optional, since the local oscillator can be used to select the spectral band of interest to be received. FIG. 3 shows a super-channel receiver consists of multiple parallel digital coherent receiver units.
Compared to individual WDM channels, a main advantage of using subcarriers is to eliminate inter-channel gap, which optimizes the bandwidth utilization.
To meet the flexibility demand, the super-channel transceiver can be designed to allow reconfiguration of various parameters, besides the regular configurable parameters such as center wavelength/frequency and power level. For example, (1) the number of subcarriers can be adjusted according to the capacity requirement and bandwidth availability; (2) the spacing between adjacent subcarriers can also be tuned dynamically; (3) the modulation format, multiplexing scheme, DSP technology, FEC (forward error correction) coding, etc. in each subcarrier can be adjusted, especially when digital transmitter is used (i.e. generate the signal electronically using DSP, convert the digital signal to analog signal, and then modulate onto an optical signal); (4) the data rate or symbol rate (also called the baud rate) of each subcarrier can be adjusted. (Some references are [Huang2], [Liu].)
These reconfiguration capability allows the user to set different channel data rate, bandwidth usage, channel characteristics, etc., so that the optical spectrum can be better utilized, transmission performance can be optimized, and cost can be minimized.
These reconfigurations can be done manually or through software controller. This type of transceiver is called variable optical transceiver here. It is a key element in software-defined optical network (SDON), which is the physical layer hardware in software-defined network (SDN), where the user can flexibly reconfigure the physical hardware using intelligent software through a common interface.
In WDM optical network application, the transceiver is a key element in a transponder, which converts signal between client (this side is called the “client side”) and the WDM network (this side is called the “WDM side” or “line side”). FIG. 4 illustrates the basic structure of a transponder with super-channel transceiver, which also consists of electronic processors and the client side transceiver, which supplies the client data to the super-channel transceiver and vice versa.
The large capacity and configuration flexibility of these variable optical super-channel transceivers also bring out some practical issues:                1. The hardware might be over-provisioning: These variable optical transceivers are designed to accommodate the maximum possible channel capacity, but often they are not stretched to the maximum, but running at some intermediate capacities. This makes the hardware usage below optimum. Also, in an optical transponder, the processor and the client side also need to have hardware to support the maximum capacity, but also often under-utilized. For example, if a transponder contains a variable optical super-channel transceiver with up to 10 subcarriers with maximum per-subcarrier capacity of 100 Gb/s, this transponder has the maximum capacity of 1 Tb/s per channel at the WDM side (also called the line side), it will also have the electronic processor to process 1 Tb/s data, as well as a client side hardware to supply 1 Tb/s data (such as 10×100 Gb/s, or 25×40 Gb/s). Depending on the dynamic traffic condition, this variable transceiver might only transmit and receive 400 Gb/s data at some moment (using only 4 subcarriers), that means that 60% of the transmitter and receiver capability is not utilized, and similarly 60% of the processing power and 60% of the client side hardware are not utilized at this time.        2. Coarse switching granularity: In the optical transport network, each super-channel is switched as one unit during transmission and optical switching, until it is converted to electrical signal for regeneration or data processing. Therefore for switching granularity is the entire super-channel capacity, which is often beyond 100 Gb/s, up to several Tb/s. This actually reduces the flexibility in the bandwidth utilization.        
Accordingly, there is a need for a variable optical transceiver that overcomes limitations of prior teachings to accommodate increasingly dynamic global data traffic over optical transport networks.